Fixation of Isotopic Nitrogen on a Semiarid Soil by Algal Crust Organisms

Abstract

Abstract Semiarid desert algal crust organisms were found to fix N 2 when exposed to an atomosphere which contained isotopically enriched N. Significant quantities of the N isotope were detected in the total crust N after 3 days of incubation under field‐simulated conditions. Net N 2 fixation rates by the algal crust organisms were 0.16 and 0.10 lb of N/acre of crust surface per day under continuous wet and cycling wet‐dry conditions, respectively. Net fixation of N under field‐simulated conditions adequately compensated for the removal of N by livestock. The rate of N 2 fixation under field‐simulated conditions increased linearly for at least 520 days. The amount of N in the algal crust was doubled during this time. No net N change was observed in dry crusts. Growing algal crusts contained 1% to 2% of the total N as extracellular NH 4 ‐N. Excretion of some fixed nitrogen was suggested by the isotopic enrichment of the extracellular N fraction and uptake of labeled N by grass seedlings ( Artemesia sp. ) growing on incubated crusts.